Modeling Bubbly Flows and Bubble-Mediated Gas Transfer in High Wind Conditions

University Of Washington, Seattle WA

Investigators

Abstract

Bubbles in the ocean are a key component of air-sea gas transfer at moderate to extreme wind speeds. They enhance gas transfer rate by providing a pathway in addition to the ocean surface. Due to surface tension and hydrostatic pressure exerted on bubbles, gases are able to dissolve at supersaturated conditions. The surface ocean is, therefore, supersaturated with outgassing at the ocean surface and interior dissolution through bubbles. The primary objectives of this study are to better understand processes governing the evolution of bubbles and dissolved gases under high winds and to improve parameterization for bubble-mediated gas flux. Three hypotheses, identified based on previous observational and theoretical studies, will be tested: (1) Gas dissolution through bubbles is important in determining mixed layer dissolved gas concentration under a hurricane; (2) Wind speed dependence for bubble penetration and bubble-mediated gas flux is smaller in high wind conditions; (3) Bubble-enhanced effective solubility for Dimethyl Sulfide and the kinetics of the carbonate system have substantial impacts on air-sea flux of these gases. The hypotheses will be tested using a coupled - large eddy simulation - bubble population - dissolved gas concentration model, which has been shown to faithfully reproduce the simultaneous evolution of and the interplay among oceanic surface boundary layer turbulence, bubbles of multiple sizes with multiple gas components, and the dissolved concentrations of multiple gases. The model also includes the bubble-induced enhancement in effective solubility for polar gas DMS and chemical reactions for the carbonate system. The model will be forced by realistic wind and wave conditions under hurricane Frances (2004), where high quality oceanic measurements of turbulence, dissolved oxygen and Nitrogen will be used to gauge the model realism, as well as steady winds. Intellectual Merit : Subsurface bubbles, especially in high wind conditions, are hitherto insufficiently understood, and the contributions of bubbles to the total air-sea gas flux are poorly constrained and have not yet been included in any existing climate simulations or biogeochemical process estimates. The synthesis of simulations and data will provide accurate description of processes governing bubbles and dissolved gases at high spatial and temporal resolutions. Results of this study will provide fundamental understanding of and mechanistically based parameterization for bubble distribution and bubble-mediated gas transfer. They will also be applicable to the air-sea transfer of other reactive and non-reactive gases. Broader Impacts : The research in bubbles and air-sea gas transfer in high wind conditions contributes broadly to the understanding of the earth system in terms of both the physical coupling of the ocean and atmosphere and the global biogeochemical cycling. Better parameterization of bubble-mediated gas flux for large scale and climate models will result in significant improvements in the modeling and budget estimate of climatically and environmentally important soluble gases including carbon dioxide, oxygen and Dimethyl Sulfide. This study will improve our capability to predict future environmental and climatic changes, and will provide better scientific basis for decision makers in politics and industry on a range of issues such as climate and energy. It, therefore, has important economic and societal implications. Although this study focuses on the impact of bubbles on air-sea gas transfer, better description of subsurface bubble fields also leads to better characterization of sound and light propagation at the surface ocean and benefits the ocean engineering community. The project will support a new investigator. Besides publishing in refereed journals, the PI will actively participate in conferences and workshops organized by established scientific groups and societies and will develop collaboration with other colleagues in the air-sea gas exchange community. Scientific results will also be presented to the general public on the PI's website. Project materials will be incorporated into the University of Washington's continual outreach effort to local schools, museums and community groups. The PI and Co-PIs will continue to be involved in outreach activities through the Seattle science festival and other volunteer activities at local schools.